Low C on a High C five string?
 

Alright. Bare with me here, I'm having a bit of a thought.

I know of a few bassists that play a five string, for access to the lower notes that sometimes pop up in some of the basso continuo lines. But, I have no need for a super low B, at most it'd be a low D or Eb.

However, I can see the benefit to having a high C string for playing into the upper register. Which is nice for solo work.

Now, I haven't any intentions in purchasing a five string bass any time in the near future. But has anyone ever thought about doing this? Using a Low-C extension on a five string with the high C string?

I saw a Czech-Ease set up like this recently. It's not my cup of tea, but some folks like it. Personally, I'm not so keen on the high C--I had one of my cheap basses modified to a 5-string, and I found that I didn't like it because the crowded bridge was hard to bow and the extra weight on the top choked the bass' sound. I decided I'd just rather practice in thumb position to get those notes...

http://www.lamario.ca/lamario1998.php

It's been done.

Well, that's something I thought I'd never see.

Again; not sure why someone would need such a thing. I was just curious to see if someone had created, or at least considered such an idea besides me.

I think I saw that bass too, along with it's twin. To be honest, the basses looked to me like a crowded mess.

5-string basses are harder to bow than 4-stringers, but it depends on the curvature of the bridge to get the best out of it.
I reshaped the bridge curvature of my 5-string bass myself, since it was a bit high and I wanted maximum and equal bridge bowing angles for each string.

Since I have a 110cm 4/4 bass with high C I was thinking about an extension, but for my use (more than 90% jazz) an extension is simply too expensive.

Note the higher lower saddle of the lamario1998, it reduces the pressure on the top, so you could make something similar for your instrument.
There are saddles available that could be made effectively higher or lower by moving a stick up and down a notched ramp. Mabe too expensive for a cheap bass, but why not ask your luthier for a price?

Since you’re in London you may want to go talk to Joe Phillips. He has two 5 stringers; one standard and one in Viennese tuning (with a high B-flat). May be worth checking out…